A. Field of the Invention
The present application relates to the field of projection television and in particular to CRT's (cathode ray tubes) used in such televisions.
B. Related Art
A projection television according to the prior art is shown in FIG. 1a. Three CRT's 101, 102, and 103 are used to produce red, green, and blue, respectively. Each CRT includes a lens assembly, where a lens is sealed to the front surface of the CRT, with fluid filling the space between the front surface and the lens. Light from the CRT's is reflected off a mirror 104 near the top of the set to produce an image on a screen 105. The green CRT 102 has an axis 106, which folds at the mirror 104, so that light on the axis 106 is incident perpendicular to the screen 105. The red and blue CRT's 101 and 103 have axes 107 and 108 which point toward the axis 106 of the green CRT.
FIG. 1b is an unfolded view of the television set of FIG. 1a.
The front surface of each CRT includes a portion around the edges which is not usable to project an image. Typically in a 7" CRT, for instance, a region having a diagonal in the range of 5" to 5.25" is usable. The raster of the CRT must therefore be restricted to the usable area. The raster approximately defines a rectangle on the green CRT and trapezoids on the blue and red CRT's. The trapezoids on the blue and red CRT's are approximately mirror images of each other. The trapezoids compensate for the angles of the axes of the CRT's. The trapezoids differ slightly in shape because of the different focal lengths of the lenses subjected to different color light and the resultant requirement for different angles of the CRT's.
The raster in each CRT, at the time the set is sold, scans an area which is slightly larger than the desired image projection area. This is necessary to increase the life of the set, because over time, as components age, the area scanned will shrink. In order to have a reasonably sized image during the expected life of the set, a slight overscan is required.
The result of the overscan is scattering of light at the edges of the CRT's and glare in the picture on the screen 105.